1. Field of the Invention
The present invention relates to an endoscope observation system that allows observation of polarized images obtained using polarized light and normal images obtained using normal light.
2. Description of the Related Art
An endoscope is conventionally known as an optical device with which the interior of a living body to be diagnosed and/or treated is observed. In general, when the appearance, shape, or the like of interior of the living body is observed using the endoscope, non polarized light is used as observation light. Illumination light is emitted to an observation target site via an illumination optical system provided inside a distal end portion of the endoscope inserted into the living body. Light from the observation target site is received by an image pickup optical system provided inside the distal end portion of the endoscope. An image pickup element then picks up an image from the light. The picked-up image is displayed on an image display device or the like.
The endoscope may be effectively used in observation using polarized light for diagnosis of, for example, HGD and an early cancer, in addition to the observation using the non polarized light. The HGD, early cancer, and the like are developed in proximity to the surface of living tissues. It is known that extracting and analyzing scattering light only from the tissue surface allows the nature of the tissues to be determined to find an abnormal tissue and that polarized light can be effectively used to extract the scattering light only from the tissue surface, as described in PCT WO 00/42912. Furthermore, polarization of light scattering inside the tissue is disturbed. The abnormal tissue such as the early cancer or the like has large cell nuclei. Consequently, the level of scattering inside the abnormal tissue is different from that inside the normal tissue. Thus, the abnormality of the interior of the tissue can be estimated by measuring the level of scattering of the polarized light (the level of disturbance of the polarized light) having entered the tissue.
PCT WO 00/42912 discloses a device shown in FIG. 1. FIG. 1 is cited from IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS VOL. 5, NO. 4, p. 1010-1026, which is authored by the inventor of PCT WO 00/42912. In the device 50, shown in FIG. 1, white light from a broadband light source 51 is guided through fibers 52 and converted into particular linearly polarized light through a lens 53, an aperture stop 54, and a polarizer 55. The linearly polarized light then enters a beam splitter 56. Light reflected by the beam splitter 56 impinges on a living tissue 57. The light is then scattered in the living tissue 57 and enters a beam splitter 56. Part of the light passes through the beam splitter 56 and then through an aperture stop 58. The part of the light is reflected by a mirror 59 and then enters a polarization beam splitter 60.
A light component of the light having entered the polarization beam splitter 60 passes through the polarization beam splitter 60; the light component has a polarization direction that is parallel to a direction in which light is polarized by the polarizer 55. The light component is guided to a multi-channel spectroscope 62 via a lens 61a or the like. A light component having a polarization direction that is orthogonal to the polarization direction of the polarizer 55 is reflected by the polarization beam splitter 60 and then guided to the multi-channel spectroscope 62 via a lens 61b or the like. The beam splitter 56 is located such that the illumination light slightly obliquely enters the living tissue 57 so as to prevent the reflected light from the living tissue 57 from directly entering the spectroscope 62. The parallel and perpendicular components separated from the incident light by the polarization beam splitter 60 enter the spectroscope 62. The spectroscope 62 subjects the components to background correction (the ratio of each of the components to a scatterer of white light is determined) and then determines the difference between the components.
This configuration irradiates the living tissue 57 with the light with the particular polarization component and divides the resultant scattering light into the polarization components that are parallel and perpendicular, respectively, to the polarization component of the irradiation light to detect spectrum. In this case, the scattering light returning from the surface of the living tissue 57 contains the polarization component that is parallel to the polarization component of the irradiation light. On the other hand, scattering light returning from a deep portion of the living tissue 57 has been intensely scattered. Thus, the scattering light contains equivalent quantities of the polarization component that is parallel to that of the irradiation light and the polarization component that is perpendicular to that of the irradiation light. Thus, the scattering light with the parallel polarization component contains both the component of the light from the surface of the living tissue 57 and the component of the light from the deep portion of the living tissue 57. The scattering light with the perpendicular polarization component contains the component of the light from the deep portion of the living tissue 57.
Here, the scattering light from the surface of the living tissue 57 can be exclusively extracted by determining the difference between the scattering light with the parallel polarization component and the scattering light with the perpendicular polarization component. Moreover, the sizes of the cell nuclei can be estimated by analyzing the spectrum of the scattering light from the surface of the living tissue 57. Thus, the use of the polarization allows the scattering light containing information on the sixes of the nuclei to be extracted at a high S/N ratio.
Besides PCT WO 00/42312, A. Harris et al., The Study of the Microcirculation using Orthogonal Polarization Spectral Imaging, Yearbook of Intensive Care and Emergency Medicine 2000 discloses a method of improving the contrast of a vessel image using polarization. Specifically, a tissue is irradiated with light with a particular polarization component. Scattering light is imaged which has a polarization component perpendicular to the polarization component of the irradiation light. In this case, the scattering light returning from the surface of the tissue contains the polarization component that is parallel to the polarization component of the irradiation light. On the other hand, scattering light returning from a deep portion of the tissue has been intensely scattered. Thus, the scattering light contains equivalent quantities of the polarization component that is parallel to that of the irradiation light and the polarization component that is perpendicular to that of the irradiation light. That is, the scattering light from the deep portion of the tissue can be imaged by imaging the light with the polarization component that is perpendicular to that of the irradiation light. This reduces the scattering light from the tissue surface, thus making an observer feel that the observer is directly viewing light from the deep portion of the tissue. The contrast of the vessel in the tissue surface can thus be improved.
As described above, for the endoscope, the observation using the polarization is expected to be more effective for determining the presence or absence of a lesion and/or diagnosis than the conventional observation with the non polarized light, for example, the observation with the normal light such as visible light. For example, Japanese Patent Laid-Open No, 2003-47588 describes a conventional endoscope observation system that provides observation images using the non polarized light such as the normal light, while providing polarized images utilizing the polarized light. FIG. 2 schematically shows the configuration of an endoscope observation system described in Japanese Patent Laid-Open No. 2003-47588. The endoscope observation system described in Japanese Patent Laid-Open No. 2003-47588 has a light source section 71, an endoscope 72 including an illumination optical system 72a and an image pickup optical system 72b, an image processing device 73, and an image display device 74.
The light source section 71 is configured to be able to emit illumination light for normal observation which allows observation images to be obtained using the normal light and illumination light for polarization observation which has a plurality of wavelength bands and which allows observation images to be obtained using the polarized light. The illumination optical system 72a also includes a polarizer 72a1 that polarizes the light from the light source section 71. The image pickup optical system 72b includes a polarization beam splitter 72b1 as a polarization separation element, and image pickup elements 72b21 and 72b22 on respective optical paths separately formed by the polarization separation element. The image processing device 73 executes predetermined image processing on image data picked up by the image pickup elements 72b21 and 72b22. The image display device 74 displays images processed by the image processing device 73. Switching the light emitted by the light source section 71 allows observation images to be obtained using the normal light or the polarized light. For normal observation, two polarized images separately formed by the polarization separation element 72b1 are synthesized via the image processing device 73.
As described above, to allow observation of the polarization condition of reflected light obtained when an observation target is irradiated with polarized light, the polarization separation element needs to be provided in the optical path in the image pickup optical system in order to obtain a polarized image. Furthermore, to allow observation of scattering light information on the mucosa using polarized light, the polarization separation element also needs to be provided in the optical path in the image pickup optical system in order to allow the following operation to be performed. As described above, the mucosa is irradiated with polarized light, and son polarized light is obtained which is a mixture of light returning from the mucosa and maintaining the polarized condition and light also returning from the mucosa but having the polarization condition disturbed. Then, the following images are picked up: an image (non polarized image) of the non polarized light, and an image (polarized image) of a particular polarization component extracted from the non polarized light. The difference between the images is then determined for analysis.